Orexin is a neuropeptide spliced from prepro-orexin, which is expressed specifically in the lateral hypothalamic area. Up to date, OX-A composed of 33 amino acids and OX-B composed of 28 amino acids have been identified, both of which are involved in the regulation of sleep-wake pattern and the regulation of feeding.
Both OX-A and OX-B act on OX receptors. Two subtypes, OX1 and OX2 receptors, of the OX receptors have been cloned so far, and both of which are known to be seven-transmembrane G protein-coupled receptors expressed mainly in the brain. OX1 receptor is coupled specifically with Gq among the G protein subclasses, whereas OX2 receptor is coupled with Gq and Gi/o (see Non Patent Literature 1 and Non Patent Literature 2). Ox receptor subtypes are selectively expressed in the brain, and OX1 receptor is expressed in high density in the locus coeruleus, which is the nuclei originis of noradrenergic neurons, whereas OX2 receptor is expressed in high density in the tuberomammillary nucleus, which is the nuclei originis of histaminergic neuron (see Non Patent Literature 3, Non Patent Literature 4 and Non Patent Literature 5). The expression of both OX1 receptor and OX2 receptor are found in the raphe nucleus, which is the nuclei originis of serotoninergic neuron, and in the ventral tegmental area, which is the nuclei originis of dopaminergic neuron (see Non Patent Literature 3). The orexin neurons project to the monoaminergic neuron system at the brain stem and the hypothalamus and have excitatory effects to these neurons, and further the expression of OX2 receptor is also found in the cholinergic neuron at the brain stem responsible for regulating REM sleep and have effects to the nucleus activities thereof (see Non Patent Literature 3 and Non Patent literature 4).
In recent years, OX1 and OX2 receptors are focused on the role of the sleep-wake regulation, and the usefulness of OX receptor antagonists have been studied. When OX-A is intracerebroventricularly administered to a rat, increased spontaneous locomotor activity (see Non Patent Literature 6 and Non Patent Literature 7), increased stereotyped behavior (see Non Patent Literature 7), increased time spent awake (see Non Patent Literature 6), and the like, were observed. Decreased REM sleep produced by OX-A administration is completely antagonized by the pretreatment of an OX receptor antagonist (see Non Patent Literature 8). Further, it is reported that locomotor activity is decreased, sleep latency is shortened, and amount of non-REM sleep and REM sleep are increased by administering an orally available OX1 and OX2 receptors antagonist (see Non Patent Literature 9 and Non Patent Literature 10). Patent Literature 1 discloses a heteroaromatic ring derivative as the compound having OX receptor antagonistic activities but does not disclose the compound having the heteroaromatic methyl cyclic amine skeleton as described in the present application. Also, compounds, for example, having various structures described in Non Patent Literature 11 are generally known as OX receptor antagonists but the compounds having the heteroaromatic methyl cyclic amine skeleton described in the present application are not disclosed.